1. Field of the Invention
This invention pertains to fuse arrays, and more particularly fuse arrays that can be fabricated on integrated circuits and electrically modified to vary their electrical and mechanical properties.
2. Description of the Related Art
A fusible link is an electrical conductor that can be melted, or broken, by ohmic heating. Fusible links, or simply, fuses, are common in consumer electronics to prevent circuit damage that can be caused by excess current. Thus, in a fused circuit, the fuse provides an electrical path within the circuit that acts as a normal conductor until a predetermined amount of excess current attempts to pass through the fuse. The excess current causes the fuse to heat excessively until the fuse fuses, i.e., melts or breaks, thereby creating an open circuit to prevent damage to other circuit components.
A polysilicon micro-fuse 2 is known in the art and shown in FIG. 12. Known as a resistive microbridge fuse, this fuse includes enlarged ends 4 coupled by a relatively thinner fuse portion 6, known as a dog bone structure. Pads 8 provide contacts for electrical connection and include a Cr/Au metal top layer. In such devices, ohmic heating, H, may be approximated as H=I2Rt, where I is current passing through the microbridge fuse, R is resistance and t is time. The current that fuses the microbridge fuse is defined by Ic=(Hc/xcfx80/R)1/2, where, Hc is the total heat flow into the fuse, xcfx84 is the thermal time and is defined as L2/xcex1pxcfx802 (where L is the fuse length and xcex1p is the thermal diffusivity of the fuse material). When I is less that Ic, the microbridge fuse conducts current indefinitely, but when I greater than Ic, the fuse blows and creates an open circuit.
Prior art circuits may incorporate many such resistance microbridge fuses to allow many identical circuits to be fabricated by one mask set and then customized by blowing or leaving fuses as desired. In order to blow a prior art fuse, each such resistance microbridge fuse requires that it be separately addressed. Thus, a circuit having 100 resistance microbridge fuses would require at least 101 electrodes, one for each fuse and one for a common ground. Further, using a common ground requires that the ground be a very large, low resistance bus so that the ground does not act as a fuse. These requirements imposes a large burden on designers and fabricators, particularly for large-scale integrated circuits.
Some prior art applications use lasers to trim fuses to customize properties for a particular circuit. Thus, a generic circuit will be fabricated having a plurality of resistors coupled to a circuit. After fabrication, the resistors may be cut by laser to modify the equivalent resistance of the plurality of resistors. While feasible, such secondary fabrication steps are expensive.
Further, these prior art fuses are only used for electrical connections. Because each fuse must be separately addressed, which requires a large number of electrodes, the use of such prior art microbridge fuses for customizing mechanical properties is impractical.
Within microelectromechanical systems (MEMS) art, mechanical resonators have application as mechanical filters for use in electrical circuits. Such mechanical filters are especially applicable as band-pass filters in high performance communication receivers, such as mobile telephones. Typical mechanical resonators that are used as filters have a plurality of resonant masses interconnected by wire couplers. The center frequency, bandwidth, and filter skirt are a function of the resonant mass size, spacing, number of elements, and mass-couplers (viz. the mass-coupler geometry).
Early mechanical filters were primarily large isolated resonators. More recently electromechanical filters have been incorporated into electrical circuits as MEMS. Because these mechanical structures are circuit mounted, they are designed and installed to operate at a specific center frequency and bandwidth. Customizing the resonate filter after incorporation into a circuit has heretofore been substantially impractical. In order to change a filter""s characteristics, a new circuit design must be fabricated. This requirement increases cost and makes prohibitively expensive the use of such mechanical filters in anything other than large scale fabrication.
In addition, even xe2x80x9cidenticalxe2x80x9d micromechanical resonators, fabricated on the same die, will have variations in their respective resonant frequencies. Thus, such micromechanical resonators must be tuned prior to use. Prior art methods of tuning such resonators include laser trimming and selective deposition.
A prior art method of fabrication of MEMS is a multi-user MEMS process (referred to as MUMPs). In general, the MUMPs process provides up to three-layers of conformal polysilicon that are etched to create a desired physical structure. The first layer, POLY 0, is coupled to a supporting nonconductive wafer. The second and third layers, POLY 1 and POLY 2, are mechanical layers that can be separated from their underlying structure by the use of sacrificial layers that separate the layers during fabrication and are removed near the end of the process. The POLY 1 and POLY 2 layers may also be fixed to the underlying structure (the wafer or lower POLY 0 or POLY 1 layer as the case may be) through openings, or vias, made by etching.
The MUMPs process also provides for a final top layer of 0.5 xcexcm thick metal for probing, bonding, electrical routing and reflective mirror surfaces.
Further information of the MUMPs process is available from Cronos Microsystems, Inc., 3021 Cornwallis Road, Research Triangle Park, N.C.
In preferred embodiments, the device of the present invention is fabricated by the MUMPs process. However, the MUMPs process may change as dictated by the Cronos Microsystems, Inc. or other design considerations. The MUMPs fabrication process is not a part of the present invention and is only one of several processes that can be used to make the present invention.
The present invention provides a fuse array for microelectromechanical systems (MEMS) that can be tuned or customized without separately addressing individual fuses. Thus, the fuse array is addressable in that any number of fuses in a sequence may be selectively blown by a common set of conductors. In particular, the present invention provides a plurality of fuses that can be addressed by two electrodes. By reducing the number of electrodes to control a plurality of fuses, circuit designs that incorporate such fuse arrays can made considerably more simple. In addition, because only two electrodes are necessary to address a plurality of fuses, such fuse arrays can be incorporated into mechanical designs and selectively blown to tune or customize mechanical properties in MEMS.
In a preferred embodiment of the present invention, a fuse array includes two conductive pads having a plurality of fuse links electrically connected in parallel between the pads. Each fuse link is a layer of polysilicon that is partially covered with a metal layer. Located along the length of the fuse link is a fuse portion that physically breaks (or blows in the vernacular) when the current in the fuse link exceeds the cutting current.
A substantially similar structure may be used in mechanical systems to customize dynamics between two or more masses. In such systems, the fuse links may be more properly referred to as fuse beams and the fuse beams are coupled to the masses, or to a single mass and a supporting structure such as a substrate.
The metal layer of the fuse links controls an electrical resistance of the link. And, as can be seen above, ohmic heating is directly proportional to the resistance and the square of the current. For a given voltage potential across the pads, the current in each link is proportional to the resistance across the respective link (Ohm""s law: V=IR). Thus, the fuse link having the most metal across the length of the link will have the least resistance, (i.e., they are the most conductive) and conduct, or pass, the most current. And because ohmic heating increases in proportion to the square of the current, the ohmic heating increases even as the resistance is reduced. If the current in the most conductive link is greater than the cutting current, the fuse in that link is blown. And, because the links are connected in parallel, the equivalent resistance of the links, that is the resistance of all the links in parallel, becomes greater. Accordingly, in order to blow the next most conductive fuse link requires that the voltage across the pads be increased. In this manner, a desired number of fuse links may be blown to create a desired resistance between the pads for customizing a circuit.
Similarly, where the fuse links are arranged exemplarily as fuse beams between masses in a resonator, a voltage may be applied across the masses to blow a desired number of fuse beams in order to tune the resonator.
Thus, the fuse array of the present invention permits fabrication of a general resonator design that can be tuned for a particular application after fabrication. Thus, resonators need not be custom designed for a particular application prior to fabrication and large scale fabrication of generic designs may be made to benefit from the economies of scale, which generic designs are later tuned for a specific application.
Thus, although resonators are know in the prior art, an obstacle to their use on circuits, and in MEMS, is the problem of tuning the resonator for a desired parameter, such as a center frequency or bandwidth where the resonator is to be used as a filter. Because the fuse array of the present invention may be fabricated as fuse beams that extend between physical masses, such fuse beams may then be used as couplings between masses and tuned by application of electrical signals. The fuse beams may be addressed and blown as are the fuse arrays, to tune, or customize, the resonator.